1. Field of the Invention
The present invention is generally in the field of fabrication of semiconductor devices. More particularly, the present invention is in the field of fabrication of heterojunction bipolar transistors.
2. Related Art
In a silicon-germanium (xe2x80x9cSiGexe2x80x9d) heterojunction bipolar transistor (xe2x80x9cHBTxe2x80x9d), a thin silicon-germanium layer is grown as the base of a bipolar transistor on a silicon wafer.
The SiGe HBT has significant advantages in speed, frequency response, and gain when compared to a conventional silicon bipolar transistor. Cutoff frequencies in excess of 100 GHz, which are comparable to the more expensive gallium-arsenide based devices, have been achieved for the SiGe HBT.
The higher gain, speed and frequency response of the SiGe HBT are possible due to certain advantages of silicon-germanium, such as a narrower band gap and reduced resistivity. These advantages make silicon-germanium devices more competitive than silicon-only devices in areas of technology where high speed and high frequency response are required.
The advantages of high speed and high frequency response discussed above require the realization of a thin highly doped base layer in the NPN SiGe HBT. For example, boron is commonly utilized to provide P-type doping of the base in an NPN silicon-germanium HBT. However, boron has a tendency to diffuse in the base. In other words, the boron profile in the base has a tendency to widen, thus undesirably widening the base. Boron diffusion is further accelerated during subsequent thermal processing steps that occur in the fabrication of the NPN SiGe HBT. The increased boron diffusion can severely degrade the high frequency performance of the NPN SiGe HBT. Thus, suppression of boron diffusion presents a major challenge in the fabrication of a NPN SiGe HBT.
One method of suppressing boron diffusion in the base of the NPN SiGe HBT is by adding carbon in the base. To effectively arrest the diffusion of boron, a heavy carbon doping level is required. For example, a concentration greater than approximately 0.1 atomic percent of carbon can be added in the base of the NPN SiGe HBT at the point where the concentration of boron peaks. Due to the high carbon concentration, the impact on the lattice is such that the periodicity of the lattice is altered to compensate total strain. Since the in-plane strain is key to band-gap narrowing in SiGe, the addition of carbon doping counters this benefit from which increased NPN performance is derived. Thus, although adding carbon in the base effectively suppresses boron diffusion, the addition of carbon has the undesirable effect of increasing the band gap in the base and consequently diminishing the performance of the NPN SiGe HBT.
Graph 100 in FIG. 1 shows exemplary boron, carbon, and germanium profiles in a base in an NPN SiGe HBT. Graph 100 includes concentration level axis 102 plotted against depth axis 104. Concentration level axis 102 shows relative concentration levels of boron, carbon and germanium. Depth axis 104 shows increasing depth into the base, starting at the top surface of the base, i.e. at the transition from emitter to base in the NPN SiGe HBT. The top surface of the base in the NPN SiGe HBT corresponds to xe2x80x9c0xe2x80x9d on depth axis 104.
Graph 100 also includes boron profile 106, which shows the concentration of boron in the base, plotted against depth, i.e. distance into the base. Boron profile 106 includes peak boron concentration level 108, which occurs at depth 114. Graph 100 further includes carbon profile 112, which shows the concentration of carbon in the base, plotted against depth. The concentration of carbon in carbon profile 112 increases abruptly from 0.0 to a constant level at depth 114, and remains at a constant level from depth 114 to depth 122. At depth 122, the carbon concentration level decreases abruptly to 0.0.
Graph 100 further includes germanium profile 116, which shows the concentration of germanium in the base of the present exemplary NPN SiGe HBT, plotted against depth. Germanium profile 116 begins at 0.0 concentration level at depth 110 and ramps up, i.e. increases linearly, to depth 118. Germanium profile 116 maintains a constant concentration level from depth 118 to depth 120. At depth 120, germanium profile 116 ramps down, i.e. decreases linearly, to 0.0 concentration level at depth 122. Thus, a concentration of carbon is added in the base of the NPN SiGe HBT at depth 114, which corresponds to peak boron concentration level 108.
Graph 200 in FIG. 2 shows an exemplary band gap curve in the base in the present exemplary NPN SiGe HBT. Graph 200 shows band gap curve 202, which shows the change in band gap caused by carbon profile 112 and germanium profile 116 in FIG. 1 in the base in the present exemplary NPN SiGe HBT. Graph 200 includes change in band gap axis 208 plotted against depth axis 204. It is noted that xe2x80x9c0xe2x80x9d on change in band gap axis 208 refers to the band gap of a reference base comprising only silicon, i.e. a silicon-only base. It is also noted that an upward move on band gap curve 202 indicates a decrease in the band gap of the base of the present exemplary NPN SiGe HBT relative to the band gap of a silicon-only base. Conversely, a downward move on band gap curve 202 indicates an increase in the band gap of the base relative to the band gap of a silicon-only base.
Depth axis 204 corresponds to depth axis 104 in FIG. 1. In particular, depths 210, 214, and 222, respectively, correspond to depths 110, 114, and 122 in FIG. 1. At depth 210, band gap curve 202 begins to decrease at a linear rate. As is known in the art, an increase in the concentration of germanium in a base of an NPN SiGe HBT results in a decrease in band gap. Thus, band gap curve 202 decreases from depth 210 to just prior to depth 214 as the result of a ramp up in concentration of germanium. At depth 214, the band gap increases abruptly from band gap level 212 to band gap level 216. This step increase in band gap corresponds to the addition of carbon in the base at depth 114 in FIG. 1. As such, the addition of carbon in the base of an NPN SiGe HBT results in an undesirable increase in the band gap of the base. This increase in band gap creates an electric field in the NPN SiGe HBT that opposes current flow, and thus results in a decrease in the speed that the NPN SiGe HBT can achieve.
Thus, there is a need in the art to provide a narrow base in a SiGe HBT by suppressing dopant diffusion in the base without causing a concomitant undesirable increase in band gap in the base.
The present invention is directed to a band gap compensated HBT. The present invention overcomes the need in the art for a narrow base in a SiGe HBT by suppressing dopant diffusion in the base without causing a concomitant undesirable increase in band gap in the base.
According to one exemplary embodiment, a heterojunction bipolar transistor comprises a base having a concentration of a first material at a first depth, where the concentration of the first material impedes the diffusion of a base dopant. For example, the first material can be carbon and the base dopant can be boron. The first material also causes a change in band gap at the first depth in the base. For example, the first material may cause an increase in band gap at the first depth in the base.
According to this exemplary embodiment, the base of the heterojunction bipolar transistor further comprises a concentration of a second material, where the concentration of the second material increases at the first depth so as to counteract the change in the band gap. For example, the second material may be germanium. The concentration of the second material, for example, may increase at the first depth by an amount required to cause a decrease in the band gap to be substantially equal to the increase in band gap caused by the concentration of the first material.